The transmission spectrum of glazing is important for the energy use of buildings, cars, greenhouses and other indoor spaces. Ideally, the visible light is transmitted in such a way that the correct light level is reached. At the same time, the infrared transmitted is controlled in such a way that the indoor temperature is controlled at the desired temperature. When too little or too much solar radiation is transmitted, much energy is needed for additional heating or cooling of the indoor space. Therefore, one would like to regulate the transmission of radiation through the window.
The most common solution to do this is to provide a non-switchable (static) coating on or in the window. This coating absorbs or reflects part of the incident radiation, thereby limiting the allowed transmission. This coating is typically a static coating and its transmission level cannot be altered. Therefore, the transmission level of the coating is optimised with a ‘best average’ solution. An additional solar control measure, such as interior or exterior sunshading is required to deal with the high-intensity situations, for example in summertime.
Alternatively, switchable coatings are available. They combine the adjustable transmission of a sunshade in a glazing element. Several technologies are available to construct a switchable coating, including electrochromic, gasochromic, suspended particle devices, polymer dispersed liquid crystals and guest-host dye systems.
Document DE 19937768 describes the use of a LCD for use as a window. The document describes how several components of a traditional liquid crystal displays are removed until one is left with two polarization filters and a homogeneous liquid crystal layer. It is generally understood that a polarization filter for an LCD is a linear absorptive polarization filter, as this is the type of filter that is found in commercial LCD's. The transmission of visible light is controlled using a liquid crystal layer between the two polarizing filters. In DE 3330305 a window system that combines two linear polarizers with a guest-host type dye system is described. The transmission ratio in this case can be tuned by means of the concentration of the dye or the thickness of the switchable layer. In both previous cases, the maximum theoretical transmittance in this case is 50%, as the polarizers maximally allow 50% of the light to pass. In practice this number is even lower due to imperfectness of the polarization filters (a maximum practical value of 35% is reported in U.S. Pat. No. 5,015,086). In many cases, <50% transmittance is too low transmittance of visible light for practical applications. Therefore, using absorptive linear polarizers is not attractive to solve this problem.
WO 99/67681 uses an approach where high light transmittance is available while still allowing (electrical) switching to lower light transmission. This is achieved by using a dichroic dye in a liquid crystal guest. By varying the concentration of the dye, the transmissive range can be tuned anywhere between virtually 100% and 0%. The downside of this approach is that the available dichroic dyes are mostly active in the visible range of the spectrum. Only a limited number of dyes is available that allow blocking of the light for a small part of the IR (infra red) spectrum (750 nm-3000 nm). Therefore, this set-up is not adequate to block the IR electromagnetic radiation.
US 2005/007506 and US 2002/0118328 describe an alternative method to provide radiation control using cholesteric liquid crystals. Following this description, the transmission level can be controlled to be 0%, 50% or 100% for a certain wavelength regime using cholesteric liquid crystal layers. In this case >50% transmission can only be achieved by switching the cholesteric layer itself. This is difficult to do in practice, especially if one would like to have a broad regime for reflection.
Document US 2008/0158448 discloses a system, in which on top and on the bottom polarizer are arranged. Between the polarizer a liquid crystal layer is sandwiched. The liquid crystal layer functions as a polarization retarding or rotating layer. The system is explicitly designed such that light transmission in the visible range (380-780 nm) is controllable [003]. In addition to this, the system in US 2008/0158448 is designed that external stimulus applied to the active layer to achieve transmittance change in the visible spectrum will have little or no effects on the filtering of UV and infrared by the light control layer. In contradiction to this, one of the benefits of the present invention is to have a switchable transmission in the visible range and in the IR range, wherein the visible light transmisison can be tuned to be more than 50% in the high-transmissive state. This cannot be achieved with polarizer based systems described in document US 2008/0158448.
Document U.S. Pat. No. 6,072,549 discloses a system for controlling light incident on a smart window. In one embodiment described in this document a liquid crystal is used. The liquid crystal is responsible for scattering the light isotropically in all directions (without applying a voltage) and to ray the light through the layer without deviation of scattering (with applying a voltage). A liquid crystal guest host system is not disclosed in document U.S. Pat. No. 6,072,549.